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Four suspicious letters that appear to have come from the same person were sent to the White House, the Pentagon campus and Sen. Ted Cruz's campaign headquarters in Houston on Monday and Tuesday, law enforcement officials told NBC News.

An envelope sent to the White House on Monday never entered the building, the Secret Service said. Also Monday, two envelopes suspected to contain ricin, according to initial testing, were detected at a mail processing center on the Pentagon campus, a Pentagon spokesman said. And on Tuesday, two people were taken to a hospital at Cruz's office after being exposed to a "white powdery substance," the Houston Fire Department told NBC DFW.

Officials told NBC News they do not consider any of the letters to be dangerous. They said they have promising leads on where the letters originated. No information on a suspect was released.

The Department of Defense allowed reporters to photograph one of their mail screening rooms. Workers wear full protective suits.

The envelope sent to the White House was addressed to President Donald Trump, the Secret Service said.

The two suspicious envelopes on the Pentagon campus were addressed to U.S. Secretary of Defense James Mattis and U.S. Chief of Naval Operations Adm. John Richardson, a source familiar with the incident told NBC News.